1. Field of the Invention
The present invention relates to a switch circuit having a plurality of branch paths.
2. Description of Related Art
As a preferable characteristic of a switch circuit (SPnT type switch circuit etc.) used for microwave/millimeter wave bands, low insertion loss characteristics and high isolation characteristics are desired, for example.
In order to satisfy these characteristics especially when an operating frequency is wide band, there is known a switch circuit (traveling wave type switch) that utilizes a distributed transmission line including a field effect transistor (FET) structure (see Japanese Patent Publications No. 2910681 and No. 3099880).
By using this traveling wave type switch, a favorable switching characteristic can be achieved in wide band. However, even when favorably configuring a traveling type switch SPDT switch using a distributed constant line having FET structure of the length of a gate electrode being 400 μm, an insertion loss is approximately 2.1 dB and an isolation characteristic is approximately 30 dB at 76 GHz. That is, it is hard to achieve enough characteristics in a millimeter wave band (approx. 30 GHz to 300 GHz).
This is because, in the distributed constant line of the traveling wave type switch, resistance value increases along with the increase of the frequency, thus seeing a branch path in OFF state from a diverging point via a transmission line, impedance cannot be shown completely open. The loss generated in the branch path in OFF state is a trouble in improving the characteristics of a switch circuit. Specifically in a conventional switch circuit, it has now been discovered that low enough loss characteristics and high enough isolation characteristic may not be achieved in a predetermined frequency band.